Compile contracts under cfg(test) for foundry tests

[maciektr]

Starknet-foundry relies on Scarb's starknet-contract target to compile contracts that can be used during test runtime. There is a use case for foundry's users to create contracts only for this purpose. Such contracts could be defined either in test files, or under cfg(test) attribute. As of now, Scarb does not have any interface that would enable foundry to compile them.
cc @MaksymilianDemitraszek

[mkaput]

I think this could lead to smearing too much abilities everywhere. Why not just establishing a pattern to write contracts in separate packages and focus on implementing #717 instead?

[maciektr]

Decided to go with #717 instead

[maciektr]

related: foundry-rs/starknet-foundry#471